Awk scans each input file for lines that match any of a set of patterns
specified literally in prog or in one or more files specified as -fprogfile. With each pattern there can be an associated action that
will be performed when a line of a file matches the pattern. Each line
is matched against the pattern portion of every pattern-action statement;
the associated action is performed for each matched pattern. The
file name - means the standard input. Any file of the form var=value
is treated as an assignment, not a filename, and is executed at the
time it would have been opened if it were a filename. The option -v
followed by var=value is an assignment to be done before prog is executed;
any number of -v options may be present. The -Ffs option
defines the input field separator to be the regular expression fs.
An input line is normally made up of fields separated by white space,
or by regular expression FS. The fields are denoted $1, $2, ..., while
$0 refers to the entire line. If FS is null, the input line is split
into one field per character.
A pattern-action statement has the form
pattern{action}
A missing {action} means print the line; a missing pattern always
matches. Pattern-action statements are separated by newlines or semicolons.
An action is a sequence of statements. A statement can be one of the
following:
if( expression ) statement [ else statement ]
while( expression ) statement
for( expression ; expression ; expression ) statement
for( var in array ) statement
do statement while( expression )
break
continue
{ [ statement ... ] }
expression # commonly var = expression
print [ expression-list ] [ > expression ]
printf format [ , expression-list ] [ > expression ]
return [ expression ]
next # skip remaining patterns on this input line
nextfile # skip rest of this file, open next, start at top
delete array[ expression ]# delete an array element
delete array # delete all elements of array
exit [ expression ] # exit immediately; status is expression
Statements are terminated by semicolons, newlines or right braces. An
empty expression-list stands for $0. String constants are quoted " ",
with the usual C escapes recognized within. Expressions take on string
or numeric values as appropriate, and are built using the operators + -* / % ^ (exponentiation), and concatenation (indicated by white space).
The operators !++-- += -= *= /= %= ^= > >= < <= == != ?: are also
available in expressions. Variables may be scalars, array elements
(denoted x[i]) or fields. Variables are initialized to the null
string. Array subscripts may be any string, not necessarily numeric;
this allows for a form of associative memory. Multiple subscripts such
as [i,j,k] are permitted; the constituents are concatenated, separated
by the value of SUBSEP.
The print statement prints its arguments on the standard output (or on
a file if >file or >>file is present or on a pipe if |cmd is present),
separated by the current output field separator, and terminated by the
output record separator. file and cmd may be literal names or parenthesized
expressions; identical string values in different statements
denote the same open file. The printf statement formats its expression
list according to the format (see printf(3)). The built-in function
close(expr) closes the file or pipe expr. The built-in function
fflush(expr) flushes any buffered output for the file or pipe expr.
The mathematical functions exp, log, sqrt, sin, cos, and atan2 are
built in. Other built-in functions:
length the length of its argument taken as a string, or of $0 if no
argument.
rand random number on (0,1)
srand sets seed for rand and returns the previous seed.
int truncates to an integer value
substr(s,m,n)
the n-character substring of s that begins at position m counted
from 1.
index(s,t)
the position in s where the string t occurs, or 0 if it does
not.
match(s,r)
the position in s where the regular expression r occurs, or 0 if
it does not. The variables RSTART and RLENGTH are set to the
position and length of the matched string.
split(s,a,fs)
splits the string s into array elements a[1], a[2], ..., a[n],
and returns n. The separation is done with the regular expression
fs or with the field separator FS if fs is not given. An
empty string as field separator splits the string into one array
element per character.
sub(r,t,s)
substitutes t for the first occurrence of the regular expression
r in the string s. If s is not given, $0 is used.
gsub same as sub except that all occurrences of the regular expression
are replaced; sub and gsub return the number of replacements.
sprintf(fmt,expr,...)
the string resulting from formatting expr ... according to the
printf(3) format fmtsystem(cmd)
executes cmd and returns its exit status
tolower(str)
returns a copy of str with all upper-case characters translated
to their corresponding lower-case equivalents.
toupper(str)
returns a copy of str with all lower-case characters translated
to their corresponding upper-case equivalents.
The ``function'' getline sets $0 to the next input record from the current
input file; getline <file sets $0 to the next record from file.
getlinex sets variable x instead. Finally, cmd| getline pipes the
output of cmd into getline; each call of getline returns the next line
of output from cmd. In all cases, getline returns 1 for a successful
input, 0 for end of file, and -1 for an error.
Patterns are arbitrary Boolean combinations (with ! || &&) of regular
expressions and relational expressions. Regular expressions are as in
egrep; see grep(1). Isolated regular expressions in a pattern apply to
the entire line. Regular expressions may also occur in relational
expressions, using the operators ~ and !~. /re/ is a constant regular
expression; any string (constant or variable) may be used as a regular
expression, except in the position of an isolated regular expression in
a pattern.
A pattern may consist of two patterns separated by a comma; in this
case, the action is performed for all lines from an occurrence of the
first pattern though an occurrence of the second.
A relational expression is one of the following:
expression matchop regular-expressionexpression relop expressionexpressioninarray-name(expr,expr,...) inarray-name
where a relop is any of the six relational operators in C, and a
matchop is either ~ (matches) or !~ (does not match). A conditional is
an arithmetic expression, a relational expression, or a Boolean combination
of these.
The special patterns BEGIN and END may be used to capture control
before the first input line is read and after the last. BEGIN and END
do not combine with other patterns.
Variable names with special meanings:
CONVFMT[Toc][Back]
conversion format used when converting numbers (default %.6g)
FS regular expression used to separate fields; also settable by
option -Ffs.NF number of fields in the current record
NR ordinal number of the current record
FNR ordinal number of the current record in the current file
FILENAME[Toc][Back]
the name of the current input file
RS input record separator (default newline)
OFS output field separator (default blank)
ORS output record separator (default newline)
OFMT output format for numbers (default %.6g)
SUBSEP separates multiple subscripts (default 034)
ARGC argument count, assignable
ARGV argument array, assignable; non-null members are taken as filenames
ENVIRON[Toc][Back]
array of environment variables; subscripts are names.
Functions may be defined (at the position of a pattern-action statement)
thus:
function foo(a, b, c) { ...; return x }
Parameters are passed by value if scalar and by reference if array
name; functions may be called recursively. Parameters are local to the
function; all other variables are global. Thus local variables may be
created by providing excess parameters in the function definition.

There are no explicit conversions between numbers and strings. To
force an expression to be treated as a number add 0 to it; to force it
to be treated as a string concatenate "" to it.
The scope rules for variables in functions are a botch; the syntax is
worse.
AWK(1)